The role of hard disk in Hikvision video recorder

Accidentally discovered the role of the hard disk in the Hikvision surveillance video recorder: Now there are 4 hard disks in a video recorder in the company, the company's monitoring review function can only be used to review the records of the previous month, remove a hard disk, and find the review records. One week is blank and cannot be selected. So to sum up, one monitoring hard disk saves records for 1 week, and 4 hard disks save records for 1 month.